There is an ongoing need for devices and methods capable of the early, passive remote detection of dangerous gases and other substances. Recently, the need for such systems has been heightened by the spread of chemical warfare technology around the world and the increasing number of acts of global terrorism. Indeed, the potential release of dangerous substances is now a serious concern not only for the military, but for local governments as well.
The development of passive remote detectors has also been driven by other factors, such as the growing concern for the effects of industrial and vehicular emissions as well as other forms of environmental pollution such as that resulting from the use of pesticides. Remote detectors are also needed to monitor and study trends in environmental conditions in order that their causes may be understood and addressed, as well as to identify and provide warnings regarding day-to-day conditions that may affect the health of local and global populations.
Passive remote detectors of chemicals in gaseous forms primarily in the atmosphere ideally operate in the 8-13.3 μm spectral range, where (a) many objects and gases at a standard temperature of about 25° C. have strong infrared (IR) emissions; (b) the atmosphere is relatively transparent over long distances; and (c) many target species have strong absorption (or emission) depending on their temperatures relative to their background features. Although the 3-5 μm spectral range may offer advantages (b) and (c), objects and gases at standard temperature do not emit significant radiation. Accordingly, a remote detector generally may effectively operate in the 3-5 μm spectral range only where alternative radiation sources such as the sun or an artificial light are available.
In the past, several types of in situ and standoff, or remote, sensor systems have been proposed. Two prominent in situ systems presently being marketed are the Chemical Agent Monitor (CAM) and the Enhanced CAM (ECAM), both of which are manufactured by Graseby Dynamics Ltd. of the United Kingdom and operate on the principle of ion mobility. Both systems are portable, with the ECAM being designed as a handheld sensor. These systems require continuous sampling of the atmosphere at the point of measurement, and therefore can detect a target gas only if the sensor is immersed in it. This is an undesirable limitation, and can be dangerous if the target gas is toxic. In addition, these systems are only capable of taking measurements in a point-wise fashion, such that numerous measurements are required to determine the boundaries of a chemical cloud.
Another commercially available system is the surface acoustic wave (SAW) minicad. The SAW system is also portable and offers the exceptionally high sensitivity of 0.2 mg/m3, but lacks specificity and cannot determine the specific identity of a gas (i.e., it can only determine any of a certain number of gases may be present). Yet another in-situ system has been proposed using a planar optical waveguide chip with 13 interferometric sensors built onto it. See “Photonic Spectra,” February, 1999, at 42. The device detects changes in the optical index of refraction of certain films that are deposited on it. These changes occur when the target gas for which each of the films is sensitive is absorbed by the films. The system permits detection of 100 parts per billion of benzene, toluene and xylene, all toxic chemicals. Like the CAM, this device is an in-situ sensor. In addition, this system can detect only the gases for which absorbing films are available.
Remote detection of chemicals depends mostly on optical techniques, which can be classified into two groups: (a) active techniques such as lidar (light detection and ranging), differential absorption lidar (DIAL), or laser-induced fluorescence (LIF) and (b) passive techniques, such as Fourier transform IR spectroscopy (FTIR), or multi or hyper spetral techniques such as gas filter correlation radiometry (GFCR) or tunable etalons. Several research groups are developing DIAL techniques for detecting chemical agents (CAs) using wavelength-agile CO2 lasers. For example, a laser airborne remote sensing (LARS) system under development by the United States Air Force has demonstrated slant path detection capabilities of atmospheric parameters from a distance of 21 km (see Highland et al., “Laser Long-Range Remote Sensing Program Experimental Results,” SPIE Proceedings, vol. 2580(1995) at 30-37) and exceptional on-ground testing detectivity of SF6 and NH3 at a distance of 2 km (see Higdon, et at., “Air Force Research Laboratory Long-Range Airborne CO2 DIAL Chemical Detection System,” Proc. 19th International Laser Radar Conference (1998) at 651-54; Senft, et al., “Chemical Detection Results From Ground Tests of an Airborne CO2 Differential Absorption Lidar System,” Proc. 19th International Laser Radar Conference (1998) at 657-60). Projected slant-path range of the system is 30 km. The DIAL and other lidar systems suffer from, among other things, a dependance on a narrowband, rapidly tunable laser system. Consequently, they are extremely complex and expensive, require highly trained personnel for operation, and provide an undesirably limited field of view (FOV) (which is delimited in such systems by the divergence of the laser beam). In addition to these disadvantages, the filter and detector components of the Air Force's LARS CO2 DIAL system must be cooled in a liquid nitrogen environment.
Typical LIF systems, like DIAL systems, include a tunable-wavelength laser and a large aperture telescope and detector system. Unlike DIAL systems, however, the signal in LIF systems is emitted by the fluorescence of the target species which is excited by the laser. Accordingly, the signal is weaker than that of the DIAL systems and the range is limited. On the other hand, the ability of LIF systems to reject fluorescence signals from unwanted background and scattered signals, and therefore the specificity of such systems, is superior to that of DIAL systems. LIF systems are generally used for detection of biological species. See Suliga, et al., “U.S. Army Chemical and Biological Defense Command's Short Range Biological Standoff Detection System (SR-BSDS),” Joint Workshop on Standoff Detection for Chemical and Biological Defense, Williamsburg, Va. (Oct. 26-30 1998), at 265-74.
A survey of passive remote sensors is provided in Hewish, “Detection and Protection: What You Don't Know Can Kill You,” Janes International Defense Review (1997) at 30-48. Perhaps the most notable currently available passive sensor is the M21 Remote Sensing Chemical Agent Alarm (RSCAAL) This system has already been fielded and is based on a FTIR technology. It can detect clouds of toxic agents from distances of up to 5 km with excellent sensitivity when the temperature difference between the gas and its surroundings is 4° K., and at a lesser sensitivity when the temperature difference is lower. The main disadvantage of FTIR systems, including the RSCAAL system, is that they depend on a complete spectral scan followed by detailed analysis of the spectrum. To accomplish that scan, the system requires a complex, highly refined mechanical tunning arrangement which is difficult to make sufficiently rugged for field applications. Operating such a complex system and analyzing its detailed output requires highly trained personnel. Furthermore, because the system covers essentially the entire spectrum, the radiation available at each spectral location is only fraction of the radiation collected during the time of the entire scan.
In remote sensor systems, hyperspectral or multispectral imaging techniques may be employed to provide an imaging capability. In hyperspectral imaging spectroscopy, spectrally and spatially resolved information is acquired to provide a two dimensional image of the distribution of chemicals targeted for detection. Hyperspectral images may be obtained by an imaging spectrometer, in which case a narrow strip in the FOV is imaged onto the front slit of the spectrometer. The dispersive element in the spectrometer creates a full spectrum for each point of the imaged line, thereby forming a two-dimensional pattern (wavelength vs. linear spatial position), which is recorded by a focal plane array (FPA) in the back plane of the spectrometer. A full hyperspectral data cube may be obtained by imaging additional strips in the FOV while recording the point-by-point spectral distribution. Alternatively, the data cube may be divided into spectral “slices,” i.e., the two-dimensional FOV may be viewed through a tunable bandpass filter that transmits one color at a time. Monochromatic images of the two-dimensional object are recorded sequentially to obtain a stack of images of the same object—each at a different wavelength.
Hyperspectral techniques may require up to 200 such images covering a wide spectral range. Multispectral techniques, on the other hand, typically cover 20 spectral slices per object. Undoubtedly, systems employing hyperspectral techniques may provide greater spectroscopic detail and therefore have higher specificity (i.e., the ability to reject interferences by species that are not targeted for detection). However, they also require longer scans and much larger data storage and processing capabilities than multispectral-based systems, and therefore are less useful for operation by untrained personnel or from fast moving platforms.
Several hyperspectral and multispectral imaging techniques for the remote detection of chemicals have been proposed. One of the truly hyperspectral detection techniques includes the use of a tunable acousto-optic filter (AOTF) for the 8-12 μm range at a bandwidth of 3 cm−1. See Suhre, et al., “Imaging Spectroradiometer for the 8-12 μm Region with a 3 cm−1 Passband Acousto-Optic Tunable Filter,” Appl. Opt., vol. 37, no. 12 (1998) at 2340-45. But biasing the acousto-optic element during the tuning process causes the image to shift slightly, thereby complicating correlation between images obtained at different wavelengths. A separate technique using a tunable Fabry-Perot etalon is also being developed for hyperspectral imaging of chemical agents. See Rossberg, “Silicon Micromachined Infrared Sensor with Tunable Wavelength Selectivity for Application in Infrared Spectroscopy,” Sensors and Actuators A 46-47 (1995) at 413-16. However, fielding a Fabry-Perot filter is complicated by the need for good alignment and spacing control between the moving mirrors. Finally, it has been proposed to use a diffractive lens as a tunable element for gaseous chemical imaging. See U.S. Pat. No. 5,479,258, issued Dec. 26, 1995 to Hinnrichs, et al., which is incorporated by reference herein in its entirety. But cross talk between its spectral images can compromise its specificity.
All three hyperspectral techniques discussed here offer the potential of high spectral resolution, broad spectral scanning capabilities and excellent radiative throughput. However, to benefit from these potential advantages, a full set of images, each at a separate wavelength must be recorded. Assuming that these techniques can uniformly provide a bandwidth of 10 cm−1, then coverage of the entire 8-13.3 μm range (1250 to 752 cm−1), the range over which many chemicals of interest are spectrally active, will require 42 separate images. The time required to acquire these images is usually limited by the maximum imaging rate of available FPAs, which presently stands at ≦50 Hz. Nearly one second will therefore be required to record an entire data cube (or longer when the bandwidth is narrower).
In contrast to hyperspectral imaging techniques, multispectral techniques like those employed by GFCR, DAR or NFCR systems cover only the spectral regions that are needed to detect the selected target species. A typical GFCR includes a sample cell containing a target species, and a reference vacuum cell. The sample cell and reference cell are moved mechanically into and out of the detector FOV. See Herget, et al., “Infrared Gas-Filter Correlation Instrument for In-Situ Measurement of Gaseous Pollutant Concentrations,” App. Opt. 15 (1976) at 1222-28. Such systems have been used to monitor smoke stack pollutants such as CO, NO, SO2, HCl, and HF by absorption of radiation from an IR source across that stack. Detection ranges of 10-5000 ppm-m have been achieved for many of these species. An alternative GFCR method for the detection of absorption by trace atmospheric species of natural IR emission, or the emission by the detected gases themselves has been used to sense CH4, C2H6, HCl, and CO. See Ward, et al., “Gas Cell Correlation Spectrometer: GASPEC,” App. Opt., 14 (1975) at 2896-904. The system has been used both in the upward and nadir looking modes through up to 300 m atmospheric paths. The specificity of the system has been demonstrated by showing experimentally that a 1000 ppm-m change in the background burden of CO2 produced the noise equivalent of the detection of 400 ppm-m of CO.
The primary drawback of GFCRs is the need for the sensor to include a cell containing the sample species, which may present a hazard when the sensor is to be used for the detection of toxic chemicals or chemicals that are difficult to handle. In addition, the requirement of a separate cell for each target chemical results in a bulky detector. Moreover, the need to mechanically switch the cells in and out of the FOV significantly reduces system reliability and speed, and may prevent the imaging of proliferated chemicals, such as gas clouds or liquid spills, due to a loss of registration between consecutive images.
U.S. Pat. No. 5,128,797, issued Jul. 7, 1992 to Sachse, et al., which is incorporated by reference herein in its entirety, proposes a non-mechanical GFCR and DAR, which uses optical polarization modulation to switch between the optical paths of the system. See also Wang, et al., “Demonstration of New GFCR Method with CH4 Measurements at 2.3 Microns, Conference of the Optical Remote Sensing of the Atmosphere Sixth Topical Meeting,” Salt Lake City, Utah (Mar. 8-12, 1993). Although this approach avoids the unreliability associated with mechanically switching between the cells, it is undesirably complex and expensive, requiring the use of a polarization modulator, two polarization beam-splitters, and a waveplate. In addition, the performance of available polarizing beam splitters in the 8-13.3 μm spectral range presently is not sufficient to develop a sensitive detector of the design of the '797 patent for use in the 8-13.3 μm range.
U.S. Pat. No. 5,905,571, issued May 18, 1999 to Butler, et al., which is incorporated by reference herein in its entirety, discloses a new GFCR-like system which uses micro-machined diffraction gratings to produce spectra that are similar to the absorption spectra produced by the sample cells of the GFCR. According to the '571 patent, it may be possible to prepare an array of gratings, each simulating the sample cell of one chemical species. By placing the gratings in front of a single detector, or a detector array, measurements may be produced that are comparable to those of the GFCR itself, but without the inherent disadvantage of using sample cells. While attempting to address this disadvantage, however, the grating-based design of the '571 patent introduces a number of additional disadvantages. Unlike standard GFCR designs, the grating-based GFCR requires that a narrow slit be positioned between the grating and the detector in order to separate radiation directed by the grating that has the desired spectral characteristics from radiation that does not have the desired characteristics. The slit is also used to reduce interferences by stray light, to which the grating-based system is particularly susceptible. Without such a slit, the system loses some of its spectral resolving capabilities. Although the detector itself may be used as a relatively wide slit, the spectral resolution of the system would be significantly reduced. Consequently, the light gathering capability or FOV of the grating-based GFCR system is limited by the slit width. A compromise therefore must be made between two conflicting requirements: a large signal or large FOV that requires a wide slit, or high spectral resolution that requires the slit to be narrow.
The slit required by the design of the '571 patent also prevents the possibility of imaging the spatial distribution of chemical clouds. In addition, because light falling on the gratings must be collimated, whereas light falling on the slit must be focused, the grating-based GFCR must include a train of optical elements consisting of collimating and focusing lenses. Given the optics required, the fact that the system is sensitive to optical alignment between the grating and the slit, and the fact that the system requires sufficient optical path for the proper grating dispersion to develop (which may require up to several centimeters of distance between the grating and slit), the grating-based GFCR system clearly is not capable of miniaturization.
U.S. Pat. No. 3,955,891, issued May 11, 1976 to Knight, et al., which is incorporated by reference herein in its entirety, discloses a similar dispersive-correlation technique. The design of the '891 patent employs a concave grating to disperse the incoming light into its various spectral components, then selects the desired components by placing a spatial filter, shaped to correlate with the pattern formed by the desired spectrum, in the focal plane of the grating. The system has many of the features and disadvantages of the grating-based GFCR. A significant additional disadvantage, however, is that the need for a combination of a concave grating and spatial filter assemblies renders the system complex and cumbersome.
U.S. Pat. No. 4,790,654, issued Dec. 13, 1988 to Clarke, which is incorporated by reference herein in its entirety, discloses an alternative dispersive, multispectral technique which uses an imaging system followed by a cylindrical lens that creates a line focus of an image, which is then projected on a planar diffraction grating. The radiation diffracted from the grating is made of numerous strips, each at a different color and each propagating at a slightly different angle. By intercepting this radiation by a segmented mirror, it is possible to control each color component independently form the others. For example, by controlling the reflectivity of certain mirror segments it is possible to remove from the image selected spectral components. Although the technique of the '654 patent purports to provide well-defined spectral signatures, as well as capabilities of imaging preprogrammed species, it is exceptionally complex and, depends heavily on fine optical alignment which can affect the spectral resolution and the registration between images that are designed to represent the species itself and images that are representative of its background.
Althouse, et al., “Chemical Vapor Detection with a Multispectral Thermal Imager,” Opt. Eng., 30 (1991) at 1725-33, disclose a multispectral technique employing a striped filter containing up to eight strips of bandpass optical filters of 0.5 μm bandwidth (or 35-70 cm−1) to cover the 8-13.3 μm range. This approach is disadvantageous, however, as it depends on an exceptionally broad bandwidth, which results in an undesirably low specificity. This is because the entire spectral range is covered by only eight bandpass filters. In addition, the Althouse, et al. design suffers from undesirably low sensitivity because it fails to recognize the need for a background subtraction and normalization technique. Althouse, et al. recommend the use of cryogenically cooled filters, and the positioning of such filters sufficiently far away from the detector that noise contributions by radiation from the filters themselves will be reduced to acceptable levels.
Wimmers, et al., “Focal Plane Arrays: Better, Smaller IR Images for New Applications,” The Photonics Design and Applications Handbook, H-212-217 (1997) discloses a technique for detecting gaseous chemicals in the 3-5 μm range, using bandpass filters. Four bandpass filters are attached to a four position filter wheel and are spun in front a detector. Typically, each filter is selected to have a transmission band that matches the absorption spectra of certain gas species. By imaging the FOV through such filters, it is possible to obtain images of the gaseous species that have an absorption that matches their corresponding filter transmission. Disadvantageously, the design requires that the filters be cryogenically cooled. In addition, the design does not provide for methods to correct for the effects of background radiation or emission, or absorption by atmospheric or other background species. Furthermore, the use of a mechanical filter wheel to switch between the filters reduces the reliability of the system and prevents miniaturization. In addition, the use of moving filters inherently leads to a loss of registration between the images obtained through the two different filters, thereby preventing the possibility of subtracting one image from the other. The Wimmers, et al. method also does not contemplate correcting for background interferences.
Lopez, et al., “Multispectral Interference Filters and Their Application to the Design of Compact Non-Dispersive Infrared Gas Analaysers for Pollution Control,” Sensors and Actuators A, 37-38 (1993) at 502-06, disclose a bandpass filter-based approach that allows simultaneous detection of multiple species. In this technique, a single substrate is coated by multiple refractive layers and then etched to produce numerous bandpass filters each at a different location on the substrate. Preferably, the filter is designed as a linear array of various bandpass filters. The transmission line-center of each filter can be selected to match the absorption or emission of a selected species. By placing the bandpass filter arrangement in front of a linear detector array, simultaneous measurements of the absorption or emission by multiple species may be obtained. However, Lopez, et al. focus solely on filter construction concepts, contemplating only the direct measurement of intensity values. Lopez, et al. do not consider the need to correct for background interference or how such correction would best be accomplished.
Accordingly, there is a need for a sensor that is capable of remotely detecting, and preferably imaging, gaseous, liquid, solid or adsorbed chemicals even when the background, its constituents and its illumination change rapidly. Such a sensor preferably would be sufficiently simple in design to be compact, rugged, inexpensive, and easy to use and interpret by untrained operators. For example, it would be desirable to have a sensor that is sufficiently compact and rugged to be configured and used as a handheld chemical detector, while having a satisfactory sensitivity, specificity, ability to correct for effects of background radiation or emission and absorption by background atmospheric or other species, and a high imaging or spatial resolution. It would also be desirable for such a sensor to be designed alternatively to have a large FOV for large area coverage, and to be sufficiently energy efficient to be useful for long-duration, stand-alone operation. Such a sensor preferably would be capable of detecting, and preferably imaging, gaseous chemicals such as dust, atmospheric effluents, pollution, pesticide vapors, naturally occurring atmospheric gases (e.g., H2O, CO2, O3, N2O, NOx, and CO gases), gas leaks, liquid spills, hydrogen and hydrocarbon fires, surface impurities, plasmas or electric discharges. Such a sensor also preferably would have an exceptionally large signal to noise ratio, thereby permitting the use of uncooled detectors in the IR or smaller imaging lens in handheld applications.